Transcript 081106Seattle_CldPhysSem_Didlake.ppt

Convective-scale Downdrafts in the Principal
Rainband of Hurricane Katrina (2005)
Anthony C. Didlake, Jr.
COGS Seminar
UW, Dept. Atmos Sci., Seattle, November 6, 2008
Idealized structure of a tropical cyclone
downwind
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Inner and Outer
eyewalls
Stationary Band
Complex (SBC)
• principal band
• secondary bands
upwind
Willoughby 1988
Overview
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Background and Motivation
Description of RAINEX and dataset
Methodology: Convective separation and
cross sections
Characteristics of downdrafts within
principal rainband
Forcing mechanisms and immediate
effects of downdrafts
Possible impacts on larger tropical
cyclone
Summary and conclusions
Background and Motivation
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Dynamic role of principal rainband in
the larger storm remains uncertain
Several modeling studies suggest the
principal rainband impacts the storm
intensity
• PV generation and inward advection
• Inhibiting inflow of warm, moist air
• Formation of secondary eyewall via vortexRossby wave dynamics
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Important to understand structure and
dynamics of principal rainband, so that
we may better address the difficulties in
forecasting tropical cyclone intensity
AugustSeptember
NSF
NOAA
NRL
NCAR
UW
Houze et al. 2006, 2007
UM
Model of Principal Rainband
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Convective cells embedded in stratiform rain
Overturning updraft, two downdrafts
Hence and Houze 2008
Downdrafts in the Principal Rainband
IED
LLD
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Low-level downdraft (LLD)
Inner-edge downdraft (IED)
Forcing mechanisms, immediate effects,
possible impacts on larger storm?
Downdrafts in ordinary convection
Zipser 1977
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Convective-scale saturated downdraft forced
by precipitation drag
Mesoscale downdraft due to evaporative
cooling
Palmén and Newton 1969, Biggerstaff and
Houze 1993, Yuter and Houze 1995
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Convective-scale downdraft
forced by buoyancy pressure
gradient force (BPGF) field
Hurricane Katrina (2005)
ELDORA data
Reflectivity at 2 km
Convective/stratiform separation
Convective
Stratiform
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Weak echo
No echo
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Based on local gradients in
reflectivity
Similar to Steiner et al. 1995,
TRMM satellite data classification
2D frequency distributions
Reflectivity data
in % of height total
2D frequency distributions
Vertical velocity data
in % of height total
Rainband cross sections
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Radial cross sections at regular angular intervals
• 0.375°  109 cross sections
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Cross section coordinates based on classification
Average vertical velocity
Reflectivity (dBZ) as black contours
IED
LLD
LLD analysis
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Reflectivity (dBZ) as black contours
Located in lower
levels
Embedded in heavy
precipitation
LLD forcing mechanism
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Reflectivity (dBZ) as black contours
Located in lower
levels
Embedded in heavy
precipitation
Zipser’s
“Convective-scale
saturated
downdraft”
Forced down by
precipitation drag
Attains negative
buoyancy from
continuous
evaporative cooling
IED analysis
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IED investigation area: 8.5 km  12.5 km
IED analysis:
Downward vertical mass flux 2D distribution
in kg s-1
IED analysis:
Conditional probability distribution of IED speeds at 1 km
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Condition: 4.5 km-IED ≤ 3 m s-1
or > 3 m s-1
Weak mid-level IED comes with
weaker low-level IED, while
strong mid-level IED comes with
stronger low-level IED
IED analysis:
Vertical velocity at 4 km
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Intermittent pattern of convective-scale updraft and
downdraft cores
IED analysis:
Autocorrelation of vertical velocity, Lag = 4 (≈ 4.5 km)
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Physical relationship between IEDs and updrafts
IED forcing mechanism
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Overlaid by in-plane wind vectors
Originates above
the melting level,
outside of heavy
precipitation
Occurs on the
convective scale,
rather than
mesoscale
IED forcing mechanism
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Reflectivity (dBZ) as black contours
Originates above
the melting level,
outside of heavy
precipitation
Occurs on the
convective scale,
rather than
mesoscale
Initially forced by
the BPGF created by
the updraft
IED forcing mechanisms
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2-step process!
Initially forced by
the BPGF created by
the updraft
IED forcing mechanisms
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2-step process!
Initially forced by
the BPGF created by
the updraft
Attains negative
buoyancy by
evaporating heavy
precipitation of
rainband
IED effects:
Sharp inner-edge reflectivity gradient
IED effects:
Sharp inner-edge reflectivity gradient
IED effects:
Low-level wind maximum (LLWM)
Tangential wind speed
Vertical velocity
Divergence
Vertical vorticity
Possible impacts:
Increased inward flux of angular momentum
Barnes et al. 1983
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LLWM lies in radial inflow
Increased angular momentum results in
stronger vortex
Conceptual model of rainband cross section
Commonly observed features of principal rainband
Hence and Houze 2008
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Barnes et al. 1983
Upwind end consists of newer, robust convective cells
Downwind end consists of older cells collapsing into
stratiform precipitation
Principal rainband is often stationary relative to the
storm center
Possible impacts:
Growth and sustenance of principal rainband
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Area of divergence near
surface under LLD
Preferred region of
convergence on upwind
side of LLD core
Growth of updraft on
upwind end of rainband
Convergence
Background flow
LLD
Plan view
Possible impacts:
Growth and sustenance of principal rainband
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Tropical storm Ophelia
(2005)
Operational radar from
Melbourne, FL
Discrete propagation of
vertical velocity cores,
rainband cells
Stationary rainband
relative to storm center
Radar loop
Conceptual model of rainband at 2 km
Conceptual model of rainband at 2 km
Conceptual model of rainband at 2 km
Conclusions
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Principal rainband contains two repeatable
convective-scale downdrafts
Low-level downdraft is forced by precipitation
drag beneath heavy precipitation
Inner-edge downdraft is initially forced by
pressure perturbations created by nearby
buoyant updrafts, then evaporative cooling
Vorticity dynamics of updraft and IED create a
low-level wind maximum that leads to increased
angular momentum of storm
Interaction between updraft and two downdrafts
leads to growing and sustaining convection of
principal rainband
Convective-scale features allow principal rainband
to continue its impact on the overall storm
Future Work
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Analyze convective-scale structures
in high-resolution model output from
RAINEX
Investigate outer rainbands and
compare to inner core of storm
Acknowledgments
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Bob Houze
Deanna Hence, Stacy Brodzik
Brad Smull, Tomislav Maric, Jian
Yuan, Mesoscale Group
Michael Bell, Sandra Yuter
Beth Tully
Atmos Grad 2006
Extra Slides
Convective/stratiform classification
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Technique used in Steiner et al. 1995, Yuter
and Houze 1997, Yuter et al. 2005
Algorithm separates convective regions from
stratiform regions by comparing local
reflectivity to background reflectivity
Tuning of algorithm required to recognize
convective regions; the rest is designated as
stratiform
Convective/stratiform classification
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Convective center if:
• Z  Zti
• Z-Zbg  Zcc(Zbg)
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Classified convective within R(Zbg) from
convective center, remaining is classified
stratiform (unless Z < Zwe)
Zti= 45 dbZ; Zwe= 20 dbZ; R = 0.5+.23(Zbg-20); Rbg= 11 km; a=9; b=45
 1 Z bg 
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Z cc  a cos
b
2
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4 km reflectivity
2 km reflectivity
2 km vertical velocity
6 km vertical velocity
Average downdrafts for upwind half
Statistical significance testing
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Two-sided Student’s t statistic
Significance level of 95%
Null hypothesis that true autocorrelation is
zero
Number of independent samples
determined by formula of Bretherton et al.
(1999)
t
r N 2
1 r
2
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N
1 r
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2
N
1 r
2
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Figure 16
Frequency of “strong” vertical velocities
Reflectivity (dBZ) as colored contours
IED
LLD